Optical lithography is a driving force behind the continual improvements in the size and performance of the integrated circuit (IC) since its inception. Feature resolution down to 0.25 xcexcm is now routine using a 248 nm KrF excimer wavelength and optical projection tools operating at numerical apertures above 0.60 with aberration levels below 0.04 xcex RMS OPD. The industry is at a point where resolution is limited for current optical lithographic technologies. In order to extend capabilities toward sub-0.25 xcexcm, modifications in source wavelength, optics, illumination, masking, and process technology are required and are getting very much attention. Off-axis illumination and phase shift photomasks contribute to extending the range of optical lithography below 0.25 microns.
However, as devices get smaller, the photomask pattern becomes finer. Fine patterns diffract light and thus detract from imaging the photomask onto the surface of a wafer. FIG. 1a shows what happens when a photomask with a fine pattern 6 having a high frequency (pitch 2d is about several microns), is illuminated through a projection lens system 7, The fine pattern 6 is illuminated along a direction perpendicular to the surface thereof and it diffracts the light that passes through the mask 6, Diffraction rays 3-5 caused by the pattern include a zero-th order diffraction ray 5 directed in the same direction as the direction of advancement of the input ray, and higher order diffraction rays such as positive and negative first order diffraction rays 3, 4, for example, directed in directions different from the input ray. Among these diffraction rays, those of particular diffraction orders such as, for example, the zero-th order diffraction ray and positive and negative first order diffraction rays 3, 5 are incident on a pupil 1 of the projection lens system 7, Then, after passing through the pupil 1, these rays are directed to an image plane of the projection lens system, whereby an image of the fine pattern 6 is formed on the image plane. In this type of image formation, the ray components, which are contributable to the contrast of the image, are higher order diffraction rays. If the frequency of a fine pattern increases, it raises a problem that an optical system does not receive higher order diffraction rays. Therefore, the contrast of the image degrades and, ultimately, the imaging itself becomes unattainable.
As will be shown below, some solutions to this problem rely upon shaping the rays of light impinging the photomask in order to compensate for the lost contrast due to diffraction. These techniques rely upon optical systems for shaping the rays that illuminate the photomask.
In considering potential strategies for sub-0.25 xcexcm lithography, the identification of purely optical issues is difficult. Historically, the Rayleigh criteria for resolution (R) and depth of focus (DOF) has been utilized to evaluate the performance of a given technology:
R=k1xcex/NA
DOF=+/xe2x88x92k2xcex/NA2
where k1, and k2 are process dependent factors, xcex is wavelength, and NA is numerical aperture. As wavelength is decreased and numerical aperture is increased, resolution capability improves. Considered along with the wavelength-linear and NA-quadratic loss in focal depth, reasonable estimates can be made for system performance. Innovations in lithography systems, materials and processes that are capable of producing improvements in resolution, focal depth, field size, and process performance are those that are considered most practical.
The control of the relative size of the illumination system numerical aperture has historically been used to optimize the performance of a lithographic projection tool. Control of this NA with respect to the projection systems objective lens NA allows for modification of spatial coherence at the mask plane, commonly referred to partial coherence. This is accomplished through specification of the condenser lens pupil size with respect to the projection lens pupil in a Kxc3x6hler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illuminator apertures. By controlling the distribution of diffraction information in the objective lens with the illuminator pupil size, maximum image modulation can be obtained.
Phase shift masking also contributes to sub 0.25-micron lithography. Prior to the work of Levenson, et. al., as reported in xe2x80x9cImproving Resolution in Photolithography with a Phase Shifting Mask,xe2x80x9d IEEE Transactions on Electron Devices, VOL., ED-29, Nov. 12, Dec. 1982, pp. 1828-1836, it was generally thought that optical lithography would not support the increased density patterning requirements for feature sizes under 0.5 microns. At this feature size, the best resolution has demanded a maximum obtainable numerical aperture (NA) of the lens systems. However, the depth of field of the lens system is inversely proportional to the NA, and since the surface of the integrated circuit could not be optically flat, good focus could not be obtained when good resolution was obtained and it appeared that the utility of optical lithography had reached its limit. The Levenson paper introduced a new phase shifting concept to the art of mask making which has made use of the concepts of destructive interference to overcome the diffraction effects. See also U.S. Pat. No. 5,702,848.
Ordinary photolithography, with diffraction effects, is illustrated in FIGS. 3(a) to 3(d). As the mask features P1 and P2 become closer, N becomes smaller, and as seen in FIG. 3(b), the light amplitude rays, which pass through P1 and P2, start to overlap due to diffraction effects. These overlapping portions result in light intensity at the wafer, FIG. 3(d), which impinges on the photoresist layer. Accordingly, due to diffraction, the intensity of the wafer no longer has a sharp contrast resolution in the region between P1 and P2.
As illustrated by FIGS. 4(a) to 4(e), it is possible to make use of the fact that light passing through the masking substrate material, FIG. 4(a), 51, (and FIG. 4(b), 51xe2x80x2) exhibits a wave characteristic such that the phase of the amplitude of the light exiting from the mask material is a function of the distance the light ray travels in the substrate material, i.e., thickness t.sub.1 and t.sub.2, By making the thickness t.sub.2 such that (nxe2x88x921)(t.sub.2) is exactly equal to xc2xd .lambda., where .lambda. is the wavelength of the light in the mask material, and n=refractive index of the added or subtracted natural material, then the amplitude of the light existing from aperture P2 is in opposite phase from the light exiting aperture P1. This is illustrated in FIG. 4(c) showing the effects of diffraction and use of interference cancellation. The photoresist material is responsive to the intensity of the light at the wafer. Since the opposite phases of light cancel where they overlap and since intensity is proportional to the square of the resultant amplitude, as seen in FIG. 4(d) contrast resolution is returned to the pattern.
FIG. 4(a) and FIG. 4(b) illustrate two different techniques for obtaining the interference phase shifting. In FIG. 4(a), the light traverses through a longer distance via deposited layer 52, In FIG. 4(b), the light in region P2 transverses through a shorter distance by virtue of an etched groove 52xe2x80x2 in the wafer 51xe2x80x2. The etched depth or shifter film thickness is designed to produce the desired 180 degree phase shift at the proper incident wavelength (for example, I-line or DUV).
Lithographic imaging for semiconductor integrated circuit (IC) device fabrication is sensitive to the lens aberrations in a projection lens. This becomes especially critical as geometries are pushed toward and below the wavelength of illumination. The use of phase-shift masking has been introduced in many forms to push the resolution limitations of optical lithography. A critical concern with phase shift masking is the influence that these approaches have on the impact of aberrations on imaging. FIG. 5 shows how a lens pupil is utilized with strong alternating phase shift masking. Here, only a certain radial portion of a lens pupil is utilized by diffraction information. Because of this, symmetrical aberrations such as defocus and spherical aberration can be significantly reduced. Variations on strong phase-shift masking can produce similar diffraction order distribution in the lens pupil. Problems will exist with any asymmetrical aberrations that may be present in a lens, including for example coma and astigmatism. These aberrations give rise to such things as image placement error and X/Y biases. Phase shift masking can cause this aberration to more strongly influence imaging.
One technique for solving the problem is to improve the lens with reduced aberrations. Current lenses exhibit very low aberration levels ( less than 0.04 waves) making significant improvement difficult. Furthermore, tools cost millions of dollars and owners expect to use them for several generations of devices. As such there is an unmet need to improve the performance of existing lens technology.
I discovered a practical solution that can be adapted to existing lithography tools and also to new tools. Prior art tools strive to provide a xe2x80x9ctop hatxe2x80x9d illumination intensity pattern. Such a pattern provides substantially uniform illumination across the entire circular aperture. Existing illumination systems are designed to reduce or eliminate variations in illumination. It is known that phase shift masking exacerbates aberrations of the projection lens. I have discovered that the increased impact of aberration on imaging introduced by phase shift masking could be reduced by controlling the intensity across the condenser lens aperture to provide a Gaussian distribution of light.
A typical photolithographic tool or apparatus includes an objective lens that projects an image from the phase shift photomask onto a photosensitized surface of a semiconductor wafer. The apparatus has an illuminator that generates a distribution of light. The illuminator projects its light onto the phase shift mask. That mask carries an object pattern. The objective lens receives and relays the pattern onto a photosensitive surface of a semiconductor wafer. The objective lens has aberrations.
Some are symmetrical (such as defocus and spherical) and others are asymmetrical (such as coma and astigmatism). The phase shift photomask can exacerbate these aberrations and especially the asymmetrical aberrations. My invention reduces the adverse effects of the phase shift photomask on the lens aberrations, especially asymmetrical aberrations. The invention applies a Gaussian-type distribution of light intensity to the beam of light from the illuminator.
The invention illuminates a phase-shift mask with a Gaussian or other similar distribution of energy. This method of illumination is superior to conventional circular illumination and reduces the influence of aberrations. Symmetrical aberration influence can be reduced over full pupil utilization, as is the case with conventional low-sigma circular illumination of a phase shift mask The influence of asymmetrical aberrations is further reduced compared to either full pupil utilization or conventional circular illumination of a phase shift mask. This superior method of illumination will result in significant reduction in image degradation, image placement error, and X/Y feature biasing.
In one embodiment of the invention a shaped illumination approach is described that allows for illumination of a photomask in a projection exposure tool. There is a desire for a flexible technique that can be incorporated into most existing or future generation projection exposure tools with a minimum amount of illumination system retrofitting. It is important that such an improvement be easily changed to allow a return to other operation conditions since it is expected that a given projection exposure system would be used for a variety of applications.
An existing illumination system is modified by adding a masking aperture in the illumination pupil plane, fabricated as an optical component reticle, patterned and dithered into a large number of elements to allow for control of the projected light distribution at the mask plane and inserted at the condenser lens pupil plane. This masking aperture comprises a translucent substrate and a masking film. The distribution of the intensity through the masking aperture in the illumination pupil plane is determined to provide optimized Gaussian-type illumination. The illumination region or regions exhibit varying intensity, which is accomplished by creating a half-tone pattern via pixelation of the masking film, thereby allowing for maximum variation in illumination beyond the simple binary (clear or opaque) situation possible with earlier aperture filtering approaches.
More specifically, one aspect of the invention includes an aperture mask for an illumination system to provide controlled Gaussian-type illumination. The aperture mask is divided into an array of elements and each element contains an array of pixels. Each of the elements is constructed with a matrix of pixels. The elements are patterned in accordance with a selected wavelength of incident light to diffract the incident light into an illumination pattern for illuminating a photomask.
The intensity is modulated by the intensity state of pixels within each element. The highest intensity element has all pixels clear or maximum intensity. Light of suitable wavelength passes through without attenuation. An element with all pixels having dark or minimum intensity attenuates or blocks all light. Elements of intensity between none (0%) and all (100%) are created by the state of the pixels in a given element. Random patterns and other patterns between elements may produce artifacts similar to moire patterns. Such artifacts are undesired. I discovered that a dithered pattern using position dependent thresholds produced illumination patterns that had little or no artifacts.
In another embodiment, the use of traditional optical components can also be used to achieve a Gaussian illumination shape. These include beamsplitters, prisms, lenses, or diffractive optical elements (DOEs).